Is TimeSpan unnecessary?

Question

EDIT 2009-Nov-04

OK, so it\'s been a little while since I first posted this question. It seems to me that many of the initial responders failed to r

Answer 1

consider this: an integer can be conceptualized as either an absolute value (the point on the number line) or a distance between values

By your logic, it isn't TimeSpan that's unecessary: rather it's DateTime that's unnecessary, and could be replaced by TimeSpan (duration since zero).

Plus there's the fact that integers have an obvious zero, whereas Dates however don't have an obvious zero; but having an obvious zero is necessary, if you want to replace "place on the number line" with "distance/span from the zero/origin".

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Edit:

A point (location on a plane) isn't the same as a vector.

They seem similar ...

• A vector (distance from origin) can represent a point
• A point (relative to the origin) can represent a vector

... however the value of the vector that's required to represent a given point will change if the origin changes.

It always makes sense to add two (relative) vectors; but, it makes no sense to add two points, except by converting those points to vectors and then adding the vectors.

The sum of two vectors is unaffected by a change in the origin, but the sum of two points would be affected by a change in the origin if you summed them by converting them to vectors and adding the vectors (because changing the origin would affect the values of those vectors).

[Replace 'point' with DateTime and 'vector' with TimeSpan in the argument above.]

I think there is a genuine difference between absolute and relative values. I'm don't know why that difference isn't more apparent in arithmetic, i.e. why 'numbers' are used seemingly interchangeably to represent both absolute and relative values.

Answer 2

There are a lot of complications in dates, for example:

• leap years
• leap seconds
• the 1582 change to the gregorian calendar
• the fact that there is no such thing as 0 years
• differences in the lengths of months

Treating Dates and TimeSpans as different things means that these kinds of issues are much less likely to confuse you in practise.

Answer 3

Just because you can define an operation doesn't mean you should. For example, one of the reasons division by zero is undefined is because defining it would require sacrificing some very useful properties of arithmetic (eg. associativity, etc).

The distinction between a timespan and a date comes down to addition. It makes sense to add two timespans, but it doesn't make sense to add two dates unless you have an arbitrary reference date. By not allowing addition of dates, you abstract away that arbitrary reference date. I don't know what date '0' is in .Net, and I've never needed to know. Isn't that nice?

Adding two dates is almost always a bug (seriously, try to think of where this makes sense outside of numerology). By introducing timespans (creating an Affine Space) you eliminate a whole class of bugs.

Answer 4

Asked the other way round: what would the benefit of weakening the type system in that regard be?

It’s all a question of cost vs. benefit and DateTime has the great benefit of reducing bugs due to illogical date/time calculations by forbidding such actions. DateTime exists for very much the same reasons that a strict type-checking system exists in the first place: to make semantic errors in the code produce compile-time messages. that notify the programmers of errors in their code.

Conversely, there’s the cost of having DateTime: zilch.

Now consider dropping DateTime. What would we gain?

To answer your question directly: “isn't TimeSpan redundant?” Absolutely not, it reduces bugs. It definitely has, for me.

Answer 5

(Speaking as a mathematician) It's because arithmetic operations on a "date" aren't closed or well defined, necessitating the need for an additional structure.

For example, January 1, 2000 - December 1, 1999 = ... ? We know there's 31 days between them, but if this were interpreted as a date, then the answer is Epoch (i.e., zero) + 31 days. This is not a valid "date" anymore.

Similarly, all the arithmetic operations on integers aren't well defined (1 / 2 has no answer in the integers .. integer math returns zero here, but 0 * 2 = 0, not 1 as you would expect). This necessitates the need for an additional structure that we call fractions.

Answer 6

Think about it conceptually. If I tell you that I'm having a party 7 days from now, is "7 days" in that sentence a date. Could I just say my party is on 7 days? Of course not, because 7 days isn't a date. One of the key ideas of object oriented programming is to represent concepts like this in the system as types. It's true that we could represent everything as an integer (and in fact, many people have and do), but in object oriented programming, we have the notion of types of items, and their behaviors and properties, and in that sense, it makes sense to have an object that expresses this.